Desarrollo de bioplásticos a partir de subproductos agroalimentarios con aplicaciones en envases y matrices de difusión

Abstract

Bioplastics are defined as plastics that are fully or partially produced from renewable raw materials. Such bioplastics are mixtures of biopolymers and plasticizers, whose characteristics resemble a thermoplastic (or thermosetting) synthetic polymer with similar roles in certain applications. In addition, they have the advantage of their biodegradability, which adds an extra value to materials developed from them. Among others, biopolymers from agricultural resources obtained from animal and plant raw materials such as proteins, lipids, polysaccharides and other compounds synthesized from living organisms are becoming an interesting alternative. In particular, plant-derived proteins are renewable raw materials that are produced by kilotonnes per year, for example, wheat gluten, soy protein, pea etc. Moreover, these biopolymers represent a great opportunity to give new value to what is often an important by product of the food industry. These bioplastics have a potentially high value in applications such as the development of intelligent packaging (including the formulation of food preservatives), as matrices controlling the diffusion of active species (drugs, antibiotics, pesticides, etc.) and superabsorbent materials (with applications in agriculture, controlling the dissemination of water for irrigation, fertilizer release and continuous rehydration ability, as fillers for hygiene products, etc.). This monograph aims to provide a scientific and technological approach to the development of protein-based bioplastics that, depending on their final uses, should exhibit improved water resistance, antimicrobial activity, water absorption-rehydration and/or controlled-released properties. This work will lead to the development of two types of materials: A) Biodegradable materials exhibiting improved water resistance for packaging, in non-food applications or with antimicrobial activity for active food packaging. To that end, different natural biocides have been incorporated to the bioplastics formulation and assessed their antimicrobial activity. B) Hydrophilic protein-based bioplastics with enhanced water absorption, rehydration and controlled-release capabilities. Specifically, bioplastics obtained should be able to carry out a controlled-release of active agents such as agricultural micronutrients or natural bioicides. This Ph.D. dissertation has been divided in four sections, which it is shortly described to follow: Section I Introduction Section II: Literature Review. In this section, general concepts about bioplastic, its composition and detailed information on their main components (proteins, plasticizers and Modifying agents) processing, market, potential application have been reviewed. In addition, basics concepts on rheological, thermal and microstructural characterization of bioplastics have been introduced. Section III: Materials and Methods. This section deals with the description of the materials used to prepare the different formulation of bioplastics studied, their main physico-chemical properties and processing. A description of the different test performed to characterize their rheological, thermal, molecular and microstructural properties has been explained. Additionally, water absorption and controlled release tests; and microbiological analysis of protein based bioplastics with antimicrobial agents is described. Section IV Results and Discussion: This part has been divided in four chapters. A short description to follow: Chapter I The first part of this chapter considers potato and rice proteins as a new source for the production of bioplastics. To this end, the processing of these proteins with glycerol as plasticizer is compared with wheat gluten based bioplastics. In addition, the effect of glycerol concentration on both rice and potato protein based bioplastics is studied. The second part of this chapter studied the thermal degradations of gluten and glycerol-gluten-based bioplastics. This thermal degradation have been simulated by models based on pseudocomponents which refer to the main gluten isolate and bioplastic compounds, i.e. water, glycerol, starch, glutenin and gliadin. Chapter II The first part of this chapter studied the incorporating wheat gluten into rice and potato protein based bioplastics is beneficial for the flexibility of the resulting material. This new material would display characteristics of both proteins, that is, the flexibility and thermomouldability of wheat gluten alongside the desirable water absorption properties of rice and/or potato proteins. This chapter also assesses the effect of material processing conditions under both batch mixing and extrusion processes. Chapter III The aim of this chapter is to report the development of new controlled-release bioplastics based on wheat gluten. The first section of this chapter is about controlled-release matrices based on glycerol, water, wheat gluten and an “active agent”, KCl (source of potassium chloride in agriculture). Lower water absorption and KCl release rates were obtained as more severe thermo-mechanical treatments were applied on the sample. On the contrary, citric acid addition to bioplastic formulations led to a remarkable decrease in the crosslinking degree of the protein network. Nonetheless, significantly higher water absorption and slower KCl release rate (both of them suitable properties for agricultural applications) have been found by adding citric acid. The second part is about the use of less hygroscopic plasticizers as a way of modifying bioplastic release/swelling properties. Bioplastics were formulated with polyethylene glycol (PEG), water and wheat gluten, in order to assess the effect of varying PEG molecular weight, of the addition of different additives and of changes in the processing conditions. Chapter IV Protein-based bioplastics have demonstrated their antimicrobial activity, with potential application in food packaging, when biocides were incorporated in their formulation. In terms of ability to inhibit growth of the selected microorganisms, the best results were obtained for samples containing cinnamaldehyde, carvacrol, white thyme, thymol, eugenol and linalol. The sample containing cinnamaldehyde produced the greatest degree of antimicrobial activity against all the microorganisms studied. Moreover, the thermal treatment during the mixing process further reduced water absorption to values below 15 wt.%. The lowest value was obtained for the bioplastic having 5 wt.% thymol, mixed at 80 °C and themomoulded at 120°C, for which water absorption dropped up to 5 wt.%. Section V: Conclusions. Section VI: References.